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A package for an optical semiconductor device includes an eyelet, a
signal lead inserted in a through hole formed in the eyelet, and sealing
glass sealing the signal lead in the through hole. The signal lead
includes a first portion, a second portion and a third portion that are
greater in diameter than the first portion and on opposite sides of the
first portion, a first tapered portion extending from the second portion
to the first portion, and a second tapered portion extending from the
third portion to the first portion. The first portion and the first and
second tapered portions are buried in the sealing glass. The total length
of a part of the second portion in the sealing glass and a part of the
third portion in the sealing glass is 0.2 mm or less.

1. A package for an optical semiconductor device, comprising: an eyelet;
a signal lead inserted in a through hole formed in the eyelet; and
sealing glass sealing the signal lead in the through hole, wherein the
signal lead includes a first portion; a second portion and a third
portion that are greater in diameter than the first portion and on
opposite sides of the first portion; and a first tapered portion
extending from the second portion to the first portion and a second
tapered portion extending from the third portion to the first portion,
wherein the first portion and the first and second tapered portions are
buried in the sealing glass, and wherein a total length of a part of the
second portion in the sealing glass and a part of the third portion in
the sealing glass is 0.2 mm or less.

2. The package as claimed in claim 1, wherein the sealing glass contains
air bubbles.

3. The package as claimed in claim 1, further comprising: another signal
lead inserted in the through hole, said another signal lead having a same
configuration as the signal lead, wherein the sealing glass further seals
said another signal lead in the through hole.

4. The package as claimed in claim 1, wherein the total length is 0 mm or
more.

5. The package as claimed in claim 1, wherein the second portion and the
third portion are partly or entirely exposed outside the sealing glass.

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is based upon and claims the benefit of priority
of the prior Japanese Patent Application No. 2016-002347, filed on Jan.
8, 2016, the entire contents of which are incorporated herein by
reference.

FIELD

[0002] A certain aspect of the embodiment discussed herein is related to
packages for an optical semiconductor device.

BACKGROUND

[0003] In optical communications, packages for an optical semiconductor
device, such as the TO-46 package defined by JEDEC (the JEDEC Solid State
Technology Association), are employed as packages for mounting a
surface-emitting laser or a photodiode. According to common packages for
an optical semiconductor device, leads (terminal parts) are inserted
through holes foLmed through the package and are sealed with glass.
Furthermore, according to packages for an optical semiconductor device,
the characteristic impedance of a lead is matched to, for example, 50
.OMEGA. per terminal to prevent a decrease in the efficiency of signal
transmission during high-speed communications.

[0004] In packages for an optical semiconductor device, spatial
restriction makes it difficult to enlarge a hole for inserting a lead to
match the impedance per terminal to 50 .OMEGA.. Therefore, consideration
is given to decreasing the dielectric constant of sealing glass and
reducing the wire diameter of a lead.

[0005] Reducing the wire diameter of a lead, however, not only makes the
lead easily bendable but also prevents an area for wire boding from being
created at the upper end of the lead. While the lead may be processed
into a so-called "nail lead" that is wider at the upper end, the small
wire diameter of the lead makes it difficult to perform stable
processing.

[0006] Thus, there is a limit to the reduction of the wire diameter of a
lead. Therefore, studies have been made of designing the shape of a lead
to reduce the wire diameter in part of the lead.

[0007] Reference may be made to, for example, Japanese Laid-open Patent
Publication No. 2009-105284 for related art.

SUMMARY

[0008] According to an aspect of the present invention, a package for an
optical semiconductor device includes an eyelet, a signal lead inserted
in a through hole foamed in the eyelet, and sealing glass sealing the
signal lead in the through hole. The signal lead includes a first
portion, a second portion and a third portion that are greater in
diameter than the first portion and on opposite sides of the first
portion, a first tapered portion extending from the second portion to the
first portion, and a second tapered portion extending from the third
portion to the first portion. The first portion and the first and second
tapered portions are buried in the sealing glass. The total length of a
part of the second portion in the sealing glass and a part of the third
portion in the sealing glass is 0.2 mm or less.

[0009] The object and advantages of the embodiment will be realized and
attained by means of the elements and combinations particularly pointed
out in the claims.

[0010] It is to be understood that both the foregoing general description
and the following detailed description are exemplary and explanatory and
not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

[0011] FIGS. 1A and 1B are diagrams depicting a package for an optical
semiconductor device according to an embodiment;

[0012] FIG. 2 is a diagram illustrating the shape of a first lead and a
second lead and the positional relationship between the first and second
leads and sealing glass;

[0013] FIGS. 3A and 3B are diagrams illustrating the shape of the first
and second leads and the positional relationship between the first and
second leads and the sealing glass;

[0014] FIGS. 4A and 4B illustrate a reflection characteristic of the
package according to the embodiment;

[0015] FIGS. 5A and 5B illustrate the reflection characteristic of the
package according to the embodiment; and

[0016] FIG. 6 is a graph illustrating the reflection characteristic of the
package according to the embodiment.

DESCRIPTION OF EMBODIMENTS

[0017] In designing the shape of a lead, it is desired to give sufficient
consideration to mass productivity. For example, forming a step in a lead
to reduce the wire diameter in part of the lead, which is feasible in the
case where cutting is employed as a processing technique, is not
practical in the case of employing stamping as a processing technique in
view of mass productivity. This is because by stamping, it is difficult
to provide the lead with a step where portions different in wire diameter
are directly adjacent to each other.

[0018] Furthermore, in designing the shape of a lead, sufficient
consideration has not been given to the reflection characteristic due to
impedance mismatch.

[0019] According to an aspect of the present invention, a package for an
optical semiconductor device with good mass productivity and a good
reflection characteristic is provided.

[0020] One or more preferred embodiments of the present invention will be
explained with reference to accompanying drawings. In the drawings, the
same element is referred to using the same reference numeral, and a
repetitive description thereof may be omitted.

[0021] First, a structure of a package for an optical semiconductor device
according to an embodiment is described with reference to FIGS. 1A and
1B. FIG. 1A is a perspective view of the package according to this
embodiment. FIG. 1B is a cross-sectional view of the package, taken along
the line A-A of FIG. 1A.

[0022] Referring to FIGS. 1A and 1B, a package 10 for an optical
semiconductor device (hereinafter, "package 10") according to this
embodiment includes an eyelet 20, leads 30, sealing glass 40, and sealing
glass 50.

[0023] The eyelet 20 is a substantially disk-shaped part, and is increased
in diameter to form a circular flange at its lower end. Part of a
peripheral surface of the eyelet 20 is depressed toward the center of the
eyelet 20 to faun a depression 20d. In a plan view, the depression 20d
has, for example, a substantial V shape, and may be used to, for example,
position a light-emitting device relative to the package 10 when mounting
the light-emitting device on the package 10.

[0024] The eyelet 20 may be formed of a metal material such as kovar (a
nickel-cobalt ferrous alloy) or an iron-nickel alloy. A surface of the
eyelet 20 may be plated. The eyelet 20 may be manufactured using, for
example, a cold forging stamping.

[0025] According to embodiments of the present invention, the term
"disk-shaped" refers to being of a substantially circular planar shape
having a predetermined thickness, irrespective of the size of thickness
relative to the diameter. The substantially circular planar shape may be
partly depressed or projecting.

[0026] The leads 30 include a first lead 31, a second lead 32, a third
lead 33, a fourth lead 34, and a fifth lead 35.

[0027] The first lead 31 and the second lead 32 are signal leads. The
first lead 31 and the second lead 32 are inserted in a through hole 20x
(elongated hole) that pierces through the eyelet 20 in its thickness
direction, with a longitudinal direction of the first and second leads 31
and 32 coinciding with the thickness direction of the eyelet 20. The
sealing glass 40 is provided around the first lead 31 and the second lead
32 to seal the first and second leads 31 and 32. The wire diameter of the
first lead 31 and the second lead 32 is as described below. The first
lead 31 and the second lead 32 have respective upper ends projecting
upward approximately 0 mm to approximately 0.05 mm from an upper surface
20a of the eyelet 20. The first lead 31 and the second lead 32 have
respective lower ends projecting downward approximately 6 mm to
approximately 20 mm from a lower surface 20b of the eyelet 20.

[0028] The first lead 31 and the second lead 32 are formed of a metal such
as kovar (a nickel-cobalt ferrous alloy) or an iron-nickel alloy. The
first lead 31 and the second lead 32 are configured to have their
respective upper ends electrically connected to, for example, a
light-emitting device to be mounted on the package 10. In the case of
mounting a light-receiving device as well on the package 10, the upper
ends of the first lead 31 and the second lead 32 may also be electrically
connected to the light-receiving device. Furthermore, the number of leads
to be connected to a light-emitting device and/or a light-receiving
device may be increased.

[0029] While the first lead 31 and the second lead 32 may be inserted
separately in independent through holes the same as the fourth lead 34
and the fifth lead 35, the first lead 31 and the second lead 32 are
inserted in the same through hole 20x to produce a space saving effect.

[0030] The third lead 33 is a ground lead. The third lead 33 may have a
wire diameter of, for example, approximately 0.35 mm. The third lead 33
is joined to the lower surface 20b of the eyelet 20 by, for example,
welding, to project downward approximately 6 mm to approximately 20 mm
from the lower surface 20b of the eyelet 20, with a longitudinal
direction of the third lead 33 coinciding with the thickness direction of
the eyelet 20. The third lead 33 is formed of a metal such as kovar (a
nickel-cobalt ferrous alloy) or an iron-nickel alloy. The third lead 33
is joined to the eyelet 20 to be electrically connected to the eyelet 20.
Accordingly, when the third lead 33 is grounded, the eyelet 20 is also
grounded.

[0031] The fourth lead 34 and the fifth lead 35 are power supply leads.
The fourth lead 34 is inserted in a through hole 20y that pierces through
the eyelet 20 in its thickness direction, with a longitudinal direction
of the fourth lead 34 coinciding with the thickness direction of the
eyelet 20. The sealing glass 50 is provided around the fourth lead 34 to
seal the fourth lead 34. The fifth lead 35 is inserted in a through hole
20z that pierces through the eyelet 20 in its thickness direction, with a
longitudinal direction of the fifth lead 35 coinciding with the thickness
direction of the eyelet 20. The sealing glass 50 is provided around the
fifth lead 35 to seal the fifth lead 35. The fourth lead 34 and the fifth
lead 35 may have a wire diameter of, for example, approximately 0.35 mm.

[0032] The fourth lead 34 and the fifth lead 35 have respective upper ends
projecting upward approximately 0 mm to approximately 0.05 mm from the
upper surface 20a of the eyelet 20. The fourth lead 34 and the fifth lead
35 have respective lower ends projecting downward approximately 6 mm to
approximately 20 mm from the lower surface 20b of the eyelet 20. The
fourth lead 34 and the fifth lead 35 are formed of a metal such as kovar.

[0033] The dielectric constant of the sealing glass 40 that seals the
first lead 31 and the second lead 32, which are signal leads, is lower
than the dielectric constant of the sealing glass 50 that seals the
fourth lead 34 and the fifth lead 35, which are power supply leads. For
example, the dielectric constant of the sealing glass 40 may be adjusted
to be lower than the dielectric constant of the sealing glass 50 by
causing the sealing glass 40 to contain air bubbles and controlling the
amount of air bubbles contained in the sealing glass 40. The dielectric
constant of the sealing glass 40 depends on the quality of a material
used for the sealing glass 40 and additives to the sealing glass 40.
Therefore, the amount of air bubbles contained in the sealing glass 40 is
suitably determined in accordance with the quality of a material used for
the sealing glass 40, etc. Containing air bubbles in the sealing glass 40
does not decrease the hermeticity of the sealing provided by the sealing
glass 40.

[0034] Next, the shape of the first lead 31 and the second lead 32 and the
positional relationship between the first and second leads 31 and 32 and
the sealing glass 40 are described with reference to FIGS. 2, 3A and 33.
While the following description is given, taking the first lead 31 as an
example as depicted in FIGS. 2, 3A and 33, the description is also
applicable to the second lead 32.

[0035] The first lead 31 includes a large-diameter portion 311, a tapered
portion 312, a small-diameter portion 313, a tapered portion 314, and a
large-diameter portion 315, which are concentrically and monolithically
formed by, for example, stamping. In the longitudinal direction of the
first lead 31, the large-diameter portions 311 and 315 are positioned on
opposite sides of the small-diameter portion 313 with the tapered
portions 312 and 314 interposed between the large-diameter portion 311
and the small-diameter portion 313 and between the large-diameter portion
315 and the small-diameter portion 313, respectively. In FIGS. 2, 3A and
3B, the boundary between adjacent portions among the large-diameter
portion 311, the tapered portion 312, the small-diameter portion 313, the
tapered portion 314, and the large-diameter portion 315 is indicated by
the dotted line for convenience of depiction.

[0036] A first (upper) end of the large-diameter portion 311 is a free end
to be connected by, for example, a wire, to a light-emitting device to be
mounted on the package 10. The tapered portion 312 extends between a
second (lower) end of the large-diameter portion 311 and a first (upper)
end of the small-diameter portion 313. The tapered portion 312 is tapered
toward the small-diameter portion 313. The tapered portion 314 extends
between a second (lower) end of the small-diameter portion 313 and a
first (upper) end of the large-diameter portion 315. The tapered portion
314 is tapered toward the small-diameter portion 313. A second (lower)
end of the large-diameter portion 315 is a free end. The large-diameter
portion 311, the tapered portion 312, the small-diameter portion 313, the
tapered portion 314, and the large-diameter portion 315 form a monolithic
structure.

[0037] Thus, the tapered portions 312 and 314 are provided at points where
the wire diameter changes, that is, the tapered portions 312 and 314 are
inserted between portions having different wire diameters, to gradually
change the wire diameter. As a result, it is possible to obtain a highly
mass-productive and reliable lead without sacrificing the reflection
characteristic. Here, it is assumed that no tapered portion is provided
at a changing point of the wire diameter so that the wire diameter
suddenly changes at the step-shaped changing point in the lead. Such a
lead may be manufactured by cutting, but is difficult to manufacture
using stamping in view of mass productivity. That is, in the case of
stamping, when forging processes such as drawing and swaging are
performed in a die (or between dies), a lubricant may deposit at the
corner (edge) of a step-shaped portion of the die to prevent a raw
material from flowing well into the die. This makes it difficult to shape
a lead as desired, so that the shape stability of manufactured leads
becomes poor. Furthermore, the load on the die increases.

[0038] Furthermore, in the case of applying a stress to a portion of a
lead where the wire diameter changes, the stress may concentrate on the
portion and cause the lead to bend. Furthermore, in the case of forming
the sealing glass 40 with different kinds of materials having different
coefficients of thermal expansion and sealing a lead with the sealing
glass 40, a stress concentrates on a portion of the sealing glass 40 near
the corner of a step-shaped portion of the lead, so that a crack or the
like is likely to be caused in the portion of the sealing glass 40. In
addition, it is likely that, in the portion of the sealing glass 40 near
the corner portion of the step-shaped portion of the lead, incorporated
air bubbles cannot escape so that air bubbles different from air bubbles
intentionally contained in the sealing glass 40 are generated as
so-called "captured air bubbles."

[0039] These problems can be solved by providing the tapered portions 312
and 314 at the changing points of the wire diameter.

[0040] The large-diameter portions 311 and 315 may have a wire diameter
of, for example, approximately 0.35 mm. The small-diameter portion 313
may have a wire diameter of, for example, approximately 0.21 mm. Each of
the tapered portions 312 and 314 may have a length L3 of, for example,
approximately 0.1 mm. The taper angle of the tapered portions 312 and 314
is preferably 45 degrees or less, and may be, for example, approximately
35 degrees.

[0041] Each of the small-diameter portion 313, the tapered portion 312,
and the tapered portion 314 is positioned in its entirety, that is,
buried, in the sealing glass 40. Furthermore, the total of a length L1 of
a part of the large-diameter portion 311 in the sealing glass 40 and a
length L2 of a part of the large-diameter portion 315 in the sealing
glass 40 is 0.2 mm or less. For example, as depicted in FIG. 3A, the
length Li may be 0.1 mm and the length L2 may be 0.1 mm. As another
example, as depicted in FIG. 3B, the length L1 may be 0 mm and the length
L2 may be 0 mm. In this case, no parts of the large-diameter portions 311
and 315 are present or buried in the sealing glass 40. That is, the
large-diameter portions 311 and 315 are exposed in their entirety outside
the sealing glass 40. FIGS. 3A and 3B, however, depict non-limiting
examples, and the length L1 and the length L2 may be any values that
satisfy the condition of L1+L2.ltoreq.0.2 mm. The length L1 and the
length L2 may be different values.

[0042] The reason to satisfy L1+L2.ltoreq.0.2 mm in the first lead 31 and
the second lead 32 is that it is possible to reduce reflections due to
impedance mismatch in the sealing glass 40 and impedance mismatch in air.
By reducing reflections due to impedance mismatch, an impedance value for
high-speed communications may be designed to be a specific value, taking
other conditions into consideration. For example, in the case of using
the first lead 31 and the second lead 32 for differential transmission,
it is possible to approximate the differential impedance to 100 .OMEGA.
(50 .OMEGA.+50 .OMEGA.). As a result, it is possible to realize the
package 10 having good transmission characteristics. Here, the "other
conditions" include the size of the through hole 20x, the wire diameter
of the first lead 31 and the second lead 32, and the dielectric constant
of the sealing glass 40.

[0043] The reason to satisfy L1+L2.ltoreq.0.2 mm is described in more
detail below through examples (simulations).

EXAMPLES

[Simulation 1]

[0044] The reflection characteristic, which is an important electrical
characteristic, is simulated with respect to the case of using the first
lead 31 and the second lead 32 having the shape as depicted in FIGS. 1A,
18 and 2 for differential transmission. The target value of the impedance
of the first lead 31 and the second lead 32 is set to 100 .OMEGA. (50
.OMEGA.+50 .OMEGA. because of use for differential transmission), and the
conditions of the first lead 31 and the second lead 32, such as
dimensions, are determined as described below.

[0045] Specifically, in the configuration as depicted in FIGS. 1A, 13 and
2, the length (thickness) of the sealing glass 40 is set to 0.9 mm, the
dielectric constant of the sealing glass 40 is set to 4.4, the width of
the through hole 20x (the dimension of the through hole 20x in a
direction perpendicular to its longitudinal direction in a plan view) is
set to 1.2 mm, the wire diameter of the large-diameter portions 311 and
315 is set to 0.35 mm, the wire diameter of the small-diameter portion
313 is set to 0.21 mm, the length L3 of the tapered portions 312 and 314
is set to 0.1 mm, and the taper angle of the tapered portions 312 and 314
is set to 35 degrees, and the reflection characteristic in the case of
varying a length L4 of the small-diameter portion 313 from 0 mm to 1.1 mm
in a stepwise manner is examined.

[0046] The same simulation is also performed with respect to an idealized
model. Here, the "idealized model" refers to a model having a stepped
shape without the tapered portions 312 and 314 between the large-diameter
portions 311 and 315 and the small-diameter portion 313. The shape of the
idealized model is ideal when the electrical characteristics are
considered, but is not suitable for actual processing using a die or dies
as described above. In contrast, the shape having the tapered portions
312 and 314 is suitable for actual processing using a die or dies.

[0047] The results are presented in FIG. 4A. The horizontal axis of FIG.
4A represents frequency (GHz), and the vertical axis of FIG. 4A
represents SDD11 (dB). SDD11 is an index that indicates a reflection
characteristic under the differential end condition. A smaller SDD11
value is more preferable at each frequency.

[0048] FIG. 4B schematically illustrates the length L4 of the simulated
small-diameter portion 313. Each model assumes that a light-emitting
device is connected to leads.

[0049] According to the actual structure, the first lead 31 and the second
lead 32 are connected to a light-emitting device 1 by bonding wires,
while in the simulation models, an analytical structure for clearly
indicating the influence exerted on the reflection characteristic by the
structural difference of the first lead 31 and the second lead 32,
omitting the influence of bonding wires on the reflection characteristic,
is employed.

[0050] Specifically, a submount 2 on which the light-emitting device 1 is
mounted is mounted on the eyelet 20.

[0051] The upper ends of the first lead 31 and the second lead 32 are
connected to a perfect conductor sheet 3, and the perfect conductor sheet
3 is connected to the light-emitting device 1. Furthermore, the
light-emitting device 1 is a perfect conductor model, and is grounded the
same as the eyelet 20. While being depicted in FIG. 4B for convenience of
description, the light-emitting device 1 is actually on the bottom side
of the cross sections depicted in FIG. 4B (the same as the cross section
of FIG. 1B) in the direction going into the plane of paper (that is, on
the center side of the eyelet 20) with the submount 2 extending in the
direction going into the plane of paper.

[0052] FIG. 4A demonstrates that the reflection characteristic improves as
the length L4 of the small-diameter portion 313 changes from 0 mm to 0.7
mm. The reflection characteristic, however, degrades when the length L4
of the small-diameter portion 313 is 1.1 mm. It is believed that this is
because the small-diameter portion 313 projects into the air so that an
increase in the differential impedance in the air becomes more dominant
to cause a high impedance mismatch.

[0053] Furthermore, FIG. 4A demonstrates that when the length L4 of the
small-diameter portion 313 is in the range of 0.5 mm to 0.7 mm with the
length of the sealing glass 40 being 0.9 mm, it is possible to obtain the
reflection characteristic close to that of the idealized model up to a
high-frequency range. Furthermore, it is understood from FIG. 4A that the
change of the reflection characteristic is subject to the difference of
the length L4 of the small-diameter portion 313 in the sealing glass 40
and that the influence of the presence or absence of the tapered portions
312 and 314 on the change of the reflection characteristic is limited.

[0054] Next, the data at a frequency of 25 GHz in the results of FIG. 4A
are plotted with respect to each length L4 of the small-diameter portion
313 in the graph of FIG. 5A. That is, the horizontal axis of the graph of
FIG. 5A represents the length L4 (mm) of the small-diameter portion 313,
and the vertical axis of the graph of FIG. 5A represents SDD11 (dB).
Furthermore, the graph of FIG. 5A also includes the data of the samples
depicted in FIG. 5B.

[0055] As indicated in FIG. 5A, the impedance mismatch is divided into
three Groups G1 through G3, and the importance of characteristic
impedance matching in each of the sealing glass 40 and air can be
confirmed. Group G1 is a group where the reflection due to impedance
mismatch in the sealing glass 40 is dominant. Group G3 is a group where
the reflection due to impedance mismatch in air is dominant. In contract,
Group G2 is a group that is lower in impedance mismatch in the sealing
glass 40 and air than Groups G1 and G3 to present a good characteristic
impedance.

[0056] Here, a threshold (the target reflection characteristic of SDD11)
for determining whether the impedance mismatch is high or low is set to
-19 dB, which is an indication of the characteristic commercially
required for packages for an optical semiconductor device.

[0057] It is found from FIG. 5A that when the threshold (the target
reflection characteristic of SDD11) is -19 dB, the length L4 of the
small-diameter portion 313 has to be 0.5 mm to 0.7 mm with respect to the
length of 0.9 mm of the sealing glass 40. Here, subtracting the total
length of 0.2 mm of the tapered portions 312 and 314 from the length of
0.9 mm of the sealing glass 40 results in a remainder of 0.7 mm.
Accordingly, it can be said that the target reflection characteristic
cannot be obtained unless the permissible length of the impedance
mismatch portion (the total length of the parts of the large-diameter
portions 311 and 315 in the sealing glass 40) is set to 0.2 mm or less.

[Simulation 2]

[0058] Next, the influence of the length of the sealing glass 40 is
examined. First, the two samples depicted in FIGS. 3A and 3B, namely a
sample in which the length of the impedance mismatch portion (the total
length of the parts of the large-diameter portions 311 and 315 in the
sealing glass 40) is 0.2 mm and a sample in which the length of the
impedance mismatch portion is 0 mm, are prepared. With respect to each
sample, the length of the sealing glass 40 is changed from 0.9 mm to 1.2
mm to 1.5 mm, and the influence of the change of the length is exampled.

[0059] The length of 1.2 mm and the length of 1.5 mm are dimensions
commonly employed for glass-sealed packages for a semiconductor device.
Furthermore, in the case of the length of 0.9 mm, 0.2 mm of the
glass-sealed portion of a package for a semiconductor device in which the
glass-sealed portion is supposed to be 1.1 mm in length is not sealed
with glass, with a view to matching the differential impedance to 100
.OMEGA. with an air layer and also increasing an area for mounting, for
example, a laser or a photodetector.

[0060] The results are presented in FIG. 6. The horizontal axis of the
graph of FIG. 6 represents frequency (GHz) and the vertical axis of the
graph of FIG. 6 represents SDD11 (dB). Furthermore, A-0.9 mm, A-1.2 mm,
and A-1.5 mm indicate the data of a group where the length of the
impedance mismatch portion is 0.2 mm (hereinafter, "Group A"), and B-0.9
mm, B-1.2 mm, and B-1.5 mm indicate the data of a group where the length
of the impedance mismatch portion is 0 mm (hereinafter, "Group B").

[0061] It can be confirmed from FIG. 6 that while the reflection
characteristic slightly differs between Group A and Group B, in general,
the reflection characteristic is hardly affected by the length of the
sealing glass 40.

[Experiment 1]

[0062] It was determined, with respect to the first lead 31 and the second
lead 32 of the shape depicted in FIGS. 1A, 1B and 2, whether air bubbles
other than those originally contained were generated near the tapered
portions 312 and 314 in the sealing glass 40 in the case of causing air
bubbles to be contained in the sealing glass 40. The experiment was
conducted with respect to ten samples, and no generation of air bubbles
was observed in any of the samples. Furthermore, no glass cracks were
caused near the tapered portions 312 and 314 in any of the samples.

[Experiment 2]

[0063] A cap with window glass was welded onto the eyelet 20 of the
package 10 as depicted in FIGS. 1A and 1B by electric resistance welding.
Air bubbles were contained in the sealing glass 40. Next, the package 10
on which the cap was welded was left in an environment of a temperature
of 121.degree. C., a humidity of 100%, and an atmospheric pressure of 2
atm for 280 hours, and the presence or absence of moisture penetration
into the cap was determined through the window glass. The experiment was
conducted with respect to ten samples, and no moisture penetration was
observed in any of the samples. That is, it was confirmed that containing
air bubbles in the sealing glass 40 does not decrease the hermeticity of
the sealing provided by the sealing glass 40.

SUMMARY

[0064] To sum up the results of Simulations 1 and 2 described above, the
following points carry weight to improve the reflection characteristic.

[0065] First, it is preferable that each of the small-diameter portion
313, the tapered portion 312, and the tapered portion 314 be buried in
the sealing glass 40.

[0066] Secondly, it is more preferable that the small-diameter portion
313, which is buried in the sealing glass 40, is longer, and it is more
preferable that a shorter part of each of the large-diameter portions 311
and 315 is in the sealing glass 40. Taking the required reflection
characteristic into consideration, the permissible total length of the
parts of the large-diameter portions 311 and 315 in the sealing glass 40
is 0.2 mm or less, independent of the length of the sealing glass 40.

[0067] Thirdly, the length of the small-diameter portion 313 buried in the
sealing glass 40 is dominant, and the influence of the presence or
absence of the tapered portions 312 and 314 is limited.

[0068] All examples and conditional language provided herein are intended
for pedagogical purposes of aiding the reader in understanding the
invention and the concepts contributed by the inventors to further the
art, and are not to be construed as limitations to such specifically
recited examples and conditions, nor does the organization of such
examples in the specification relate to a showing of the superiority or
inferiority of the invention.

[0069] Although one or more embodiments of the present invention have been
described in detail, it should be understood that the various changes,
substitutions, and alterations could be made hereto without departing
from the spirit and scope of the invention.

[0070] For example, while two signal leads are inserted in a single hole
according to the above-described embodiment, a single signal lead may
alternatively be inserted in a single hole. Furthermore, while two signal
leads are used for differential transmission according to the
above-described embodiment, two signal leads may alternatively be used
independent of each other. In addition, the number of leads included in a
package for an optical semiconductor device may be determined as desired.
In any of these cases, it is possible to achieve a good reflection
characteristic by satisfying the above-described conditions.